Many electrical and electronic devices are designed and constructed to operate from power at a substantially constant voltage and include a power supply to convert power from an alternating current or battery input to the required nominal voltage. However, to achieve high efficiency of such power conversion, power supplies employ switching circuits which control frequency and/or duty cycle of current pulses drawn from a power source to achieve the desired voltage with good regulation and thus inherently generate electromagnetic interference (EMI) noise at the input from the power source. EMI noise may also be generated by fluctuations in the amount on power drawn by the electrical or electronic device such as the large current swings that may be produced by a data processing or logic array circuit as switching is performed in a highly parallel and clocked fashion between periods in a stand-by state during which comparatively little power is drawn. This EMI noise reflected to the power input is referred to as conducted EMI noise or, simply, conducted EMI since it is conducted back to the power source.
Conducted EMI noise has detrimental effects on operation of electrical and electronic products, particularly due to the high frequencies present therein which can cause heating in batteries or cause fluctuations in power delivered to other devices over commercial power distribution networks which may cause improper or unintended operation thereof. Accordingly the Federal Communications Commission (FCC) issues EMI standards for almost all electronic products that may be connected either directly or indirectly to power grids and which specify the maximum conducted EMI noise level that can be produced in a wide frequency band from 10 KHz to 30 MHZ. To meet these standards, most electronic products use EMI suppression circuits such as filters to attenuate the EMI noise allowed to reach the power source. However, EMI noise is the vector sum of common mode (CM) and differential mode (DM) noise components which are not easily separated and which are often addressed in different ways in design of a filter to reduce them. EMI noise is conventionally measured using a spectrum analyzer and a pair of line impedance stabilization networks (one in each side of the power connection) which has no capability of separation of CM and DM noise components or even determining which component is dominant. Thus, EMI suppression circuit design as well as power supply circuit design has, in the past, been largely a matter of trial and error since CM and DM noise may be generated by different mechanisms and may require different approaches to EMI suppression.
Some circuits for noise separation have been proposed but none can accurately separate CM and DM noise components. Further, proposed circuits all include parasitic capacitances and inductances which, at high frequencies, necessarily degrade any degree of noise separation that any particular noise separator circuit proposed to date can achieve.